Generalize atomic/mutable lock patterns.

[toots]

This PR generalizes the new, fastest locking patterns to source's mutexify methods:

• atomic_lock is a fast, atomic-based lock. Anything using it is expected to terminate very fast. Waiting on this lock is active, done via Domain.relax_cpu
• mutable_lock is a slow, mutex-based lock. It should be used for slow operations. Waiting on this lock is done via a Atomic.wait and mutex.

Also, input.srt had some unnecessary waiting patterns on Strings.Mutable which is always protected by a similar dual mechanism.